Use a tension meter. Guessing is just not good enough. Frequency based tension meters are the only thing to use on these very lightweight synchronous belts. On my machine when new the belts recorded a frequency of around 45Hz.

Phone apps such as those for guitar tuning, spoke tensioning and general spectrum analysis are often able to measure with sufficient resolution if a dedicated tension meter is not available.

Charles, too expensive just for the occasional use on a printer! The cheapest is around $500 and the top of the range CLAVIS meter is around $2000. CLAVIS a company from England invented the principle of using frequency to measure belt tension. However, the printer belts are easy to force into vibration so a phone app. will probably work just fine.

The tension of the long belts cannot be adjusted. I think the name of that guide is wrong, it's an assembly guide, not an adjustment guide. If you have had your belts off, then you need to ensure that the teeth of the belt-ends will go into the right grooves in the cover plate. The cover plate is the small plate on top of the slider block. It has grooves in it, corresponding to the belts 2 mm tooth pitch:

If your belt hits wrong grooves then your belt will either be too loose or far too tight (I don't even think you can manage to get it misplaced in the position where it's too tight). The "two-man job" mentioned in the Raise3D guide is where my tool http://forum.raise3d.com/viewtopic.php?f=4&t=1341 will help you a lot. Just slide the ends of the belt into the tool, then you have two hands free to do work with the cover plate.If you think your belts are too loose, then I guess you need to add a small "belt tightening clip" to the belt exactly opposite the slider block, when the printer head is in its center position. There are many belt tightening clips available. I don't think there should be a need for this though. The belts are very stable and we have CNC precision machined gantry in our printers.

No, that is false. Timing belts are made to not stretch, not change with temp. They use fibers in the backing of the belt to prevent this. You should not be adjusting belts. The fact you are or thinking about it is a problem in and of itself. If a timing belt stretches, then the teeth do not line up and you get periodic jumping on the pulley.

The tool is useful when you don't listen to the above, remove the wrong screws and have the belt pop loose or some other reasons as a user that you messed up. I'm not denying that the tool is useful, it's great, but people need to STOP messing with the belts. Again, thankful this tool was developed, but let's be honest here about what happens. New or novice user screws up, loosens the screws holding a belt cap and bam, the belt shoots loose. User panics and tried for hours to fix it. Now, without this tool, it's hard for someone who knows what they are doing to tension the belt AND keep the printer in original alignment. But remember, you didn't plan ahead to print this tool first, your printer is apart right now and you are trying to fix it. You cannot print this tool because you already messed up.

Again, great tool, great idea, probably something to print now and stick in a box in the corner for if you ever need it. Pray that you never need it and if you do it was because you didn't listen and insisted on messing with the belts.

Last edited by Jetguy on Sun Jan 29, 2017 2:49 pm, edited 1 time in total.

The only exception to the rule is N1. This is because N1 uses a short closed loop belt between the X and Y motors and the upper motion/linear shafts. This machine, when you first get it after shipping, checking the tension of the short belt and adjusting the motor mounting position if required to correct makes sense. But that is a one time deal. If you have to do it ever more than once, something is very wrong.

The issue here is the how the gantry design works. If you users go changing or increasing belt tension, you in fact flex the long rods that are both the linear motion slide and the rotation to drive the other side of the printer belt system. Those rods are ONLY supported at the the very tips by bearings. The pulley is well inside the space as an overhung load. This is why there is a fixed tension system in place, not something adjustable. The position where the bearings pockets are is milled at the factory to achieve a perfect and squarely aligned frame box. The distances are all known, then resulting GT2 belt on 20t pulleys is a known distance and tension when placed. If your printer is way out of whack, one of a few very bad things has happened.#1 you've managed to bend or flex the liner motion shafts#2 your frame is so bent, that the giant one piece milled frame has shifted the bearing position allowing the rods to not be parallel and that would cause other problems#3 You've snapped the cords inside the belt. Generally, they are so strong, the belt would bend the rods long before that ever happens. The rubber of the belt has zero strength so if even one cord breaks, you are going to know it because the belt will more or less break and rip and fall off.#4 One of the belt caps came loose- either by you the user making a mistake or the screws vibrating out.

Again, I cannot stress enough, what you might use or understand from another printer or industry can cause issues here with the gantry of the N series. Because the very design has long rods with bearings only at the ends, and the belts pull well inside of that mounting point in an overhung load. So belt tension is a function of how much flex and bind you are putting in the very rods of the system. Too tight and you will put the entire machine in a bind as the sliders try to move down the now the flexed shaft.

May I reitterate what Jetguy said: If your alignment or tension is off on the N2+ the rods will bend and may bend a LOT. My pulleys came misaligned by about 1cm. I measured the X axis rod bend to be 4-5mm at the center and boy did it bind. When I aligned the pulleys you could see it jump back in place. I love good steel!

This comes up #1 on Google, so I thought I'd throw my two cents in now that I've got a decently aged machine - recently I've had issues with the extruder catching on the print (even with the second extruder removed); the belt tension had obviously degraded and upon checking the tightness of the various screws in the steel assemblies, they varied from nice and snug to loose as [insert inappropriateness here]. The steel structure was noticeably deformed as to be sagging under the weight of the extruder head.

After tightening up the collets at the end of the rods and tightening the belts appropriately (using a modified version of the above - zip ties instead of the pliers as it was just me), the structure straightened nicely back out and I was able to tune the belts to approximately the sound I remember the machine making originally.

Print accuracy has been back to great ever since; no issues with the extruder head catching, the steel structure is now clearly true to the eye and when comparing to the factory-leveled bed, and it's been back to making the harmonics it originally made when new.

SO, all that is to say that the source of the belt loosening may not be in the belts themselves - this is a fairly involved device with a number of mechanisms which could contribute to various modes of failure.